DESCRIPTION (Adapted from applicant's description): The experiments proposed in this application will take advantage of the unique opportunities afforded by a new experimental paradigm for study of the first suckling episode where inexperienced, cesarean-delivered rat pups are presented with milk made available through a surrogate nipple in a controlled experimental situation apart from active maternal care. The surrogate nipple paradigm will be used in conjunction with manipulations of endogenous activity at mu or kappa opioid receptors in the brain of the newborn rat. Antagonist drugs specific to mu (CTOP) or kappa (nor-BNI) opioid receptors will be injected into the cisterna magna or cerebral ventricles of the newborn rat to block endogenous activity at opioid receptors in rostral and caudal parts of the CNS. Measures of attachment to and ingestion of milk from the surrogate nipple, as well as milk-induced attachment to an empty nipple, will serve as behavioral assays to assess the involvement of opioid systems in the initiation, regulation and plasticity of early suckling behavior. The experiments will provide information about the integrated output of the development of the nervous system and, specifically provide insights into the newborn's first experience with milk and other suckling stimuli and into how these experiences have lasting consequences for neurobehavioral development.